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How & Why Do Leaves Change Color?

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Despite their astonishing record of losses when dealing with lumberjacks and beavers, trees are pretty tough customers. Their trunks, branches, roots and twigs are all more than capable of enduring a winter's worth of freezing temperatures, snow, sleet and hail. Their leaves, though? Eh, not so tough. The broad, thin leaves of a broadleaf tree (like a maple, an oak, a birch, or a poplar) are an Achilles' heel when winter comes, and are vulnerable to freezing and damage from the elements. In order to survive, the trees either have to somehow protect the delicate leaves or shed them.

Evergreen trees—your pines, spruces, firs, etc.— went the protection route. Their leaves, or needles, are covered in a waxy coating to resist freezing, allowing them to live for years or even decades before falling off and being replaced. The leaves of deciduous trees, on the other hand, are cast off with the arrival of winter. The chemical processes that prepare them for their send-off also treat us to the season's vibrant colors.

Color Coding

Green: The green color of leaves throughout spring and summer comes from chlorophyll, a pigment vital to photosynthesis.

As we get closer to autumn and some parts of the planet get fewer hours of sunlight, trees respond by stopping the food-making photosynthesis process and slowing the production of chlorophyll until, eventually, they stop producing it altogether and the green color of the leaf fades

leaves-mapleYellow and Orange: Along with chlorophyll, there are yellow and orange pigments, carotene and xanthophyll, inside some trees' leaves. For most of the year, these pigments are masked by chlorophyll, but as the chlorophyll breaks down and the green color dissipates, the yellow to orange colors become visible.

Red: Another class of pigment that occurs in leaves is the anthocyanins. Anthocyanins, unlike carotene and xanthophyll, are not present in leaves year-round. It isn't until the chlorophyll begins breaking down that the plant begins to synthesize anthocyanin. Why do trees begin producing a different pigment in leaves they're getting ready to lose? The prevailing theory is that anthocyanins protect leaves from sun damage, lower their freezing point, allow them to remain on the tree longer, and buy the tree more time to recover nutrients from its leaves. The colors that anthocyanins produce are dependent on the pH of the leaves' cell sap. Very acidic sap results in a bright red color, while less acidic sap leads to a purplish red.

Brown: The humdrum color is the result of waste products trapped in the leaves.

That covers the basics of how each of the colors can be produced. But which color we ultimately see depends on several factors, such as"¦

Species: Certain colors are characteristic of particular tree species and can be used to help identify the type of tree you're looking at. Oak leaves turn red, brown, or russet, hickories turn golden bronze, poplars turn golden yellow, dogwoods turn a purplish red, beeches turn a light yellow/tan, birches turn bright yellow, sugar maples turn orange-red, black maples turn a glowing yellow, and red maples turn scarlet. Some trees, notably elms, don't go through much color change at all; there's just a dull brown and then the leaf is gone with the wind.

Weather: The temperature and moisture levels a tree is exposed to before and during the time its leaves' chlorophyll breaks down can affect color. Sunny days and cool nights favor anthocyanin production and bright red leaves. On cloudy days, anthocyanin isn't as chemically active and allows the orange or yellow pigments to take center stage.

Geography: Autumn leaves in Europe tend to be mostly yellow, but the US and East Asia seem to favor red leaves. Scientists from Israel and Finland recently put forth a theory about this color difference in the journal New Phytologist1. The scientists think that some 35 million years ago—amid a series of ice ages—many tree species evolved to become deciduous and produced red leaves to ward off insects. In North America and Asia, north-to-south mountain chains enabled the north and south spread of plants and animals corresponding with the advance and retreat of ice. In Europe, east-to-west mountain ranges like the Alps trapped plant and animal life. Many tree species (and the insects that depended on them) died out when the ice advanced. At the end of repeated ice ages, say the scientists, the tree species that survived didn't need red leaves to cope with the insects that were left, so they stopped producing red pigments and stuck with yellow.

The Dead Leaves and the Dirty Ground

leaves-ground

While all this color changing and autumn magic is going on, the tree is preparing to cast off its leaves. Around the same time that chlorophyll production slows down, the veins that transport nutrients and water to the leaf from the rest of the tree get closed off. A layer of cells at the base of the leaf stem, called the separation layer, swells and forms a cork-like material, gradually severing the tissue that connects the leaf to the branch. The leaf falls off and the tree seals the cut—so when the leaf is blown off or falls from its own weight, a leaf scar is left behind.

1Lev-Yadun, S and Holopainen, J. (2009). Why red-dominated autumn leaves in America and yellow-dominated autumn leaves in Northern Europe? New Phytologist Volume 183(3): 506-512. doi:10.1111/j.1469-8137.2009.02904.x
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Special thanks to Damian Dockery, who provided the foliage photos. See more of his work at flickr.com/damiand23.

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Big Questions
Why Do Cats Freak Out After Pooping?
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Cats often exhibit some very peculiar behavior, from getting into deadly combat situations with their own tail to pouncing on unsuspecting humans. Among their most curious habits: running from their litter box like a greyhound after moving their bowels. Are they running from their own fecal matter? Has waste elimination prompted a sense of euphoria?

Experts—if anyone is said to qualify as an expert in post-poop moods—aren’t exactly sure, but they’ve presented a number of entertaining theories. From a biological standpoint, some animal behaviorists suspect that a cat bolting after a deposit might stem from fears that a predator could track them based on the smell of their waste. But researchers are quick to note that they haven’t observed cats run from their BMs in the wild.

Biology also has a little bit to do with another theory, which postulates that cats used to getting their rear ends licked by their mother after defecating as kittens are showing off their independence by sprinting away, their butts having taken on self-cleaning properties in adulthood.

Not convinced? You might find another idea more plausible: Both humans and cats have a vagus nerve running from their brain stem. In both species, the nerve can be stimulated by defecation, leading to a pleasurable sensation and what some have labeled “poo-phoria,” or post-poop elation. In running, the cat may simply be working off excess energy brought on by stimulation of the nerve.

Less interesting is the notion that notoriously hygienic cats may simply want to shake off excess litter or fecal matter by running a 100-meter dash, or that a digestive problem has led to some discomfort they’re attempting to flee from. The fact is, so little research has been done in the field of pooping cat mania that there’s no universally accepted answer. Like so much of what makes cats tick, a definitive motivation will have to remain a mystery.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
Why Do Baseball Managers Wear Uniforms?
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Basketball and hockey coaches wear business suits on the sidelines. Football coaches wear team-branded shirts and jackets and often ill-fitting pleated khakis. Why are baseball managers the only guys who wear the same outfit as their players?

According to John Thorn, the official historian of Major League Baseball since 2011, it goes back to the earliest days of the game. Back then, the person known as the manager was the business manager: the guy who kept the books in order and the road trips on schedule. Meanwhile, the guy we call the manager today, the one who arranges the roster and decides when to pull a pitcher, was known as the captain. In addition to managing the team on the field, he was usually also on the team as a player. For many years, the “manager” wore a player’s uniform simply because he was a player. There were also a few captains who didn’t play for the team and stuck to making decisions in the dugout, and they usually wore suits.

With the passing of time, it became less common for the captain to play, and on most teams they took on strictly managerial roles. Instead of suits proliferating throughout America’s dugouts, though, non-playing captains largely hung on to the tradition of wearing a player's uniform. By the early to mid 20th century, wearing the uniform was the norm for managers, with a few notable exceptions. The Philadelphia Athletics’s Connie Mack and the Brooklyn Dodgers’s Burt Shotton continued to wear suits and ties to games long after it fell out of favor (though Shotton sometimes liked to layer a team jacket on top of his street clothes). Once those two retired, it’s been uniforms as far as the eye can see.

The adherence to the uniform among managers in the second half of the 20th century leads some people to think that MLB mandates it, but a look through the official major league rules [PDF] doesn’t turn up much on a manager’s dress. Rule 1.11(a) (1) says that “All players on a team shall wear uniforms identical in color, trim and style, and all players’ uniforms shall include minimal six-inch numbers on their backs" and rule 2.00 states that a coach is a "team member in uniform appointed by the manager to perform such duties as the manager may designate, such as but not limited to acting as base coach."

While Rule 2.00 gives a rundown of the manager’s role and some rules that apply to them, it doesn’t specify that they’re uniformed. Further down, Rule 3.15 says that "No person shall be allowed on the playing field during a game except players and coaches in uniform, managers, news photographers authorized by the home team, umpires, officers of the law in uniform and watchmen or other employees of the home club." Again, nothing about the managers being uniformed.

All that said, Rule 2.00 defines the bench or dugout as “the seating facilities reserved for players, substitutes and other team members in uniform when they are not actively engaged on the playing field," and makes no exceptions for managers or anyone else. While the managers’ duds are never addressed anywhere else, this definition does seem to necessitate, in a roundabout way, that managers wear a uniform—at least if they want to have access to the dugout. And, really, where else would they sit?

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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